1. Field of the Invention
The present invention relates to a differential protective relay apparatus which differentially protects a multiple-branch bus bar having a plurality of terminals through current transformers each provided on the terminals respectively.
2. Description of the Prior Art
FIG. 1 shows a connection diagram of a power system to which a differential protective relay apparatus is applied. In the figure, indicated by 0 is a bus bar, 1 to n are terminals of the bus bar 0, 11 to 1n are current transformers (will be termed CTs hereinafter) provided for the terminals 1 to n, 20 is a differential circuit which is a parallel connection of the secondary circuits of the CT11 to CT1n, and Z.sub.D is the impedance of a differential relay 87 connected between terminals 20-1 and 20-2 of the differential circuit 20.
Generally, differential protective relay apparatus are categorized into two types of a high-impedance differential system and low-impedance differential system depending on the selected value of a high or low impedance Z.sub.D between the terminals of the differential circuit 20.
In the former high-impedance differential system, the differential circuit is shunted by a relatively high impedance Z.sub.D, and therefore it takes a little shunted current components from the current transformers CT11 to CT1n with differential connection and a little transferred energy from them. Accordingly, when currents flow in the same direction toward the differential circuit 20 on an internal fault, a relatively high voltage appears across the terminals of the differential relay 87 of the differential circuit 20. On the other hand, in case currents circulate through the CT11 to CT1n in differential connection on an external fault, voltage drops across the lead wire resistances of the secondary circuits of the CT11 to CT1n in differential connection are applied to the excitation impedance of the external fault current flow-out CT, and the terminal voltage does not exceed a certain voltage value determined by the CT excitation characteristics.
The low-impedance differential system introduces a great amount of shunted current components from the CT11 to CT1n with differential connection to the differential relay 87 of impedance Z.sub.D, and most of energy is transferred to the differential circuit. Accordingly, an internal fault does not result in the induction of a high voltage across the differential relay 87 of the differential circuit 20. On the other hand, upon application of the voltage drops across lead wire resistances of the secondary circuits of the CT11 to CT1n at an external fault, the impedance Z.sub.D of the differential circuit 20 becomes equal to or lower than the secondary excitation impedance of CT1n of the external fault current flow-out terminal, resulting possibly greater flow-in current to the differential relay 87 of the differential circuit 20. On this account, the low impedance differential system is prone to malfunction on an external fault current.
The former high-impedance differential system will further be examined in the following. Generally, assuming that R.sub.D is a resistance of a differential circuit and R.sub.2 is a total resistance the secondary circuits of CTs (secondary winding resistance R.sub.S plus secondary lead wire resistance R.sub.L of CTs), the maximum external fault current I.sub.FE max causes an apparent differential circuit current I.sub.D and differential circuit voltage V.sub.D as follows. ##EQU1##
If R.sub.D &gt;&gt;R.sub.2, the differential circuit voltage becomes: EQU V.sub.D .ltoreq.R.sub.2 I.sub.FE max ... (1.3)
and it does not exceed a certain voltage value.
On an internal fault, the minimum internal fault pick-up current is given in terms of the voltage V.sub.S appearing across the impedance Z.sub.D, the secondary excitation current I.sub.ex (V.sub.S) for the applied voltage V.sub.S, and the number n of terminals connected to the bus bars, as follows. ##EQU2##
The conventional high-impedance differential protective relay schemes have the foregoing arrangement and operation, involve the following problem to be overcome. Although, with the intention of preventing the malfunctioning, the differential relay can be set to a value lower than that given by the formula (1.3) on external faults, the minimum pick-up current on internal faults is limited to the value given by the formula (1.4) as long as the relay being set in compliance with the formula (1.3). Namely, when a large number of terminals are connected to the bus bar O and the internal fault current is small, the minimum pick-up sensitivity of fault detection adversely varies by both the secondary excitation characteristics I.sub.ex -V.sub.ex of the CTs and the number n of terminals.